The formyl peptide receptor (FPR) family belongs to the seven transmembrane domain G-protein-coupled receptor (GPCR) family. This family includes 3 members in humans, and one member of this family, FPR2 (also known as FPRL-1, ALXA4), is expressed predominantly on inflammatory cells such as monocytes and neutrophils, as well as on T cells, and has been shown to play a critical role in leukocyte trafficking during inflammation and human pathology (Chiang N, Serhan C N, Dahlen, S, Drazen J M, Hay D W P, Rovati E, Shimizu T, Yokomizo T, Brink, C. The lipoxin receptor ALX: Potent ligand-specific and stereoselective actions in vivo. Pharmacological Reviews 2006; 58: 463-519). FPR2 is an exceptionally promiscuous receptor that responds to a menagerie of structurally diverse exogenous and endogenous ligands, including serum amyloid A (SAA), chemokine variant sCKβ8-1, the neuroprotective peptide humanin, anti-inflammatory eicosanoid lipoxin A4 (LXA4) and glucocorticoid-modulated protein annexin A1 (Chiang N, Serhan C N, Dahlen, S, Drazen J M, Hay D W P, Rovati E, Shimizu T, Yokomizo T, Brink, C. The lipoxin receptor ALX: Potent ligand-specific and stereoselective actions in vivo. Pharmacological Reviews 2006; 58: 463-519). FPR2 has been shown to transduce anti-inflammatory effects of arachidonic acid derived Lipoxin A4 (LXA4) in many systems, and has been shown to play a key role in the resolution of inflammation (Dufton N, Perretti M. Therapeutic anti-inflammatory potential of formyl peptide receptor agonists. Pharmacology & Therapeutics 2010; 127: 175-188). FPR2 knockout mice show exaggerated inflammation in disease conditions as expected by the biological role of the receptor (Dufton N, Hannon R, Brancaleone V, Dalli J, Patel H B, Gray M, D'Aquisto F, Buckingham J C, Perretti M, Flower R J. Anti-inflammatory role of the murine formyl-peptide receptor 2: Ligand-specific effects on leukocyte responses and experimental inflammation. Journal of Immunology 2010; 184: 2611-2619. Gavins FNE, Hughes E L, Buss N A P S, Holloway P M, Getting S J, Buckingham J C. Leukocyte recruitment in the brain in sepsis: involvement of the annexin1 FPR2/ALX anti-inflammatory system. FASEB 2012; 26: 1-13).
Activation of FPR2 by lipoxin A4 or its analogs and by Annexin I protein has been shown to result in anti-inflammatory activity by promoting active resolution of inflammation which involves inhibition of polymorphonuclear neutrophils (PMNs) and eosinophils migration and also stimulating monocyte migration enabling clearance of apoptotic cells from the site of inflammation in a nonphlogistic manner (Gavins F N E, Hughes E L, Buss N A P S, Holloway P M, Getting S J, Buckingham J C. Leukocyte recruitment in the brain in sepsis: involvement of the annexin1 FPR2/ALX anti-inflammatory system. FASEB 2012; 26: 1-13, Madema P, Cottell D C, Toivonen T, Dufton N, Dalli J, Perretti M, Godson C. FPR2/ALX receptor expression and internalization are critical for lipoxin A4 and annexin-derived peptide-stimulated phagocytosis. FASEB 2010; 24: 4240-4249). In addition, FPR2 has been shown to inhibit natural killer (NK) cytotoxicity and promote activation of T cells which further contributes to down regulation of tissue damaging inflammatory signals.
FPR2 interaction with LXA4 and Annexin has been shown to be beneficial in experimental models of dermal inflammation, angiogenesis, epithelial migration, edema, alopecia, ischemia reperfusion and ocular inflammation, such as endotoxin-induced uveitis and corneal wound healing. (Reville K, Cream J K, Vivers S, Dransfield I, Godson C. Lipoxin A4 redistributes Myosin IIA and Cdc42 in macrophages: Implications for phagocytosis of apoptotic leukocytes. Journal of Immunology 2006; 176: 1878-1888; Serhan C. Resolution phase of inflammation: Novel endogenous anti-inflammatory and proresolving lipid mediators and pathways. Annual reviews of Immunology 2007; 25: 101-137; Medeiros R, Rodrigues G B, Figueiredo C P, Rodrigues E B, Grumman A Jr, Menezes-de-Lima O Jr, Passos G F, Calixto J B. Molecular mechanisms of topical anti-inflammatory effects of lipoxin A(4) in endotoxin-induced uveitis. Molecular Pharmacology 2008; 74: 154-161; Gronert K, Maheshwari N, Khan N, Hassan I R, Dunn M, Schwartzmann M L. A role for the mouse 12/15-lipoxygenase pathways in promoting epithelial wound healing and host defense. Journal of Biological Chemistry 2005; 280: 15267-15278; Gronert K. Lipoxins in the eye and their role in wound healing. Prostaglandins, Leukotrienes and Essential fatty Acids. 2005; 73: 221-229); Takano T, Fiore S, Maddox J F, Brady H R, Petasis N A, Serhan C N. Aspirin-triggered 15-epi-lipoxin A4 and LXA4 stable analogues are potent inhibitors of acute inflammation: evidence for anti-inflammatory receptors. Journal of Experimental Medicine 1997; 185: 1693-1704; Leoni G, Alam A, Neumann P A, Lambeth J D, Cheng G, McCoy J, Hilgarth R S, Kundu K, Murthy N, Kusters D, Reutelingsperger C, Perretti M, Parkos C A, Neish A S, Nusrat A. Annexin A1, formyl peptide receptor, and NOX1 orchestrate epithelial repair. Journal of Clinical Investigation. 2013; 123:443-54; Leedom A, Sullivan A B, Dong B, Lau D, Gronert K. Endogenous LXA4 circuits are determinants of pathological angiogenesis in response to chronic injury. American Journal of Pathology 2010; 176: 74-84; Tsuruki T, Takahata K, Yoshikawa M. Mechanism of the protective effect of intraperitoneally administered agonists for formyl peptide receptors against chemotherapy-induced alopecia. Biosci Biotechnology Biochemistry. 2007; 71:1198-202).
Pharmaceutical utility of lipoxin A4 and its analogs are hampered by inherent physicochemical properties of the natural poly-olefinic natural product. Therefore, small molecule anti-inflammatory agonists of FPR2 would have a wide variety of therapeutic benefit in inflammatory disorders, including inflammatory disorders in the eye. Targeting FPR2 selectively would also have benefits of reduced side effects as compared to more broad acting anti-inflammatories such as steroids or NSAIDs which have significant side effects of elevated IOP and delays in wound healing in the eye. FPR2 is also expressed in ocular tissues in the cornea and also the posterior of eye, in addition to the inflammatory cells that migrate into the ocular tissues.
Targeting FPR2 selectively would also have benefits in skin wound healing given its potent anti-inflammatory and pro-epithelial repair role. In addition, some skin diseases have been shown to have an abnormal expression of LL37, a pro-inflammatory cathelicidin which has been shown to be a natural ligand of FPR2. In the chronic inflammatory disease rosacea, LL37 is highly expressed and is believed to play a key role in the pathogenesis (Yamasaki K, Di Nardo A, Bardan A, Murakami M, Ohtake T, Coda A, Dorschner R A, Bonnart C, Descargues P, Hovnanian A, Morhenn V B, Gallo R L. Increased serine protease activity and cathelicidin promotes skin inflammation in rosacea. Nature Medicine. 2007; 13:975-80).
FPR2 thus represents an important novel pro-resolutionary molecular target for the development of new therapeutic agents in diseases or conditions with excessive inflammatory responses.